As energy shortages are becoming more and more serious and the calling for the environmental protection is becoming louder, the electric vehicles as a low-carbon, clean transportation, is highly concerned by governments worldwide. With the popularity of electric vehicles, the large-scale electric vehicles require to access the power grid for charging, which will have the noticeable influence on the operation of power grid. From the perspective of the power grid, with changes in requirements of urban development, there are long-distance, high-voltage transmission lines for connecting the load center with power plants, which may lead to the voltage security issues in the power grid. There are thresholds for transmission powers in the transmission system, too much energy demand of the receiving-end power grid will pose a threat to the safe operation of power grid. The load margin evaluation and control of the power grid are always concerned by dispatching personnel during the power grid operation. If the large-scale electric vehicles access to the power grid, charging powers of a large number of electric vehicles will aggravate the voltage security issues. Therefore, the voltage security evaluation and monitoring of the power grid in consideration of the access of large-scale electric vehicles should get more attention by electricity workers.
In recent decades, there are a number of voltage collapse accidents, resulting in serious economic losses. The control method responding to the voltage security which is widely applied is the under-voltage load shedding technology. However, this technology has the following defects: Firstly, this technology cannot realize a preventive control, and thus cannot carry out preventive measures which avoid the decline of power grid voltage in the case of hidden troubles existing in the power grid; secondly, this technology enables the power grid to be out of the dangerous state by way of load shedding, but it is difficult to restore the shed load, so that the control is not economic.